Partially purified leukemia-containing fractions derived from the concentrated supernatants of mouse and human long-term lymphoblastoid cell lines were used to treat DBA/2 mice bearing ascites L1210 tumors. Effective treatment was achieved by substantially reducing tumor load with a single dose of 1-(2-cloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (MeCCNU) followed by aggressive lymphokine therapy. Therapeutic effect was noted by (1) increases in median survival time (MST), (2) percent increases in life span (% ILS), and (3) increased resistance to sequentially higher challenge tumor doses of 10 to the 5th power minus 10 to the 7th power tumor cells ("cured" animals). Observations were confirmed in two strains of L1210 tumor, the former propagated at the CRC and the latter obtained from the Division of Cancer Treatment of the National Cancer Institute, U.S.A. After adjustment of proper drug dosage to LD to the 10the power levels, cures could be obtained only with combined drug lymphokine therapy. Experiments were designed to study the cells responding to lymphokines and fractions in vivo. Spleen cells from mice injected with single doses of lymphokines demonstrated enhanced cytotoxic activity toward L1210 target cells in in vitro assays. Characterization of these spleen cells by flow cytofluorometry with propidium iodide staining suggested that the responding populations showed an increased DNA turnover and an increased propensity to undergo blastogenesis. Those results suggest that an early reduction of large tumor burden followed by lymphokine administration stimulates non-specific lymphomonocyte cytotoxicity resulting in further tumor destruction to less than 1 cell and concomitant development of specific lymphocyte-mediated tumor immunity as indicated by resistance to challenge in "cured" animals.